Mixed content is elements with both text and markup children:

XML::Simple does not keep the order of the children, in this case the 2 contents ('This is ' and ' content') and the b.

You can try with this:

The output is:

<opt b="mixed">
  <content>This is </content>
  <content> content</content>
</opt>
[download]

Even if you use the noattr option you will still get the 2 contents and the b element in the wrong order.